Some thought it was a sign of alien life, while others believed it to be a weather phenomenon.
But there's a far more prosaic explanation for the appearance of a mysterious smoke ring over the M62 last weekend.
Passing drivers described the formation as 'the weirdest thing' they had ever seen when they spotted it hovering next to the motorway.
But the organisers of a nearby military re-enactment event have admitted that the ring came from their pyrotechnic display.
Several explosions were staged during World War One and Two-themed battle scenes at the Yorkshire Wartime Experience weekend in Cleckheaton.
"Our pyrotechnics team created the rings as a bit of an extra thing for the crowd, but because there was no wind they didn't disperse as normal," explained event organiser Sarah Clarke.
"The World War Two one in particular was very big, and it just floated across. There will be more of the same next year!"
Several British visitor attractions, including Warwick Castle and Legoland Windsor, have also confused nearby residents by producing strange formations linked to pyrotechnic weaponry displays.